In order to identify proteins required for the mitosis-to-interphase transition in fission yeast several temperature sensitive (ts) lethal mutants that are unable to execute this transition have been isolated. One such mutant is defective in, piml, a guanine nucleotide exchange factor for the spil GTPase. Interestingly, overexpression of spil rescues the lethality of a mutation in piml. Another mutant, temporarily named A4, does not have a mutation in piml, but like the original liml mutant, is suppressed by the spil GTPase. This suggests that the A4 gene likely to be an additional component of the spil pathway. The goal of this proposed study is to understand the role of the A4 protein in control of the cell cycle in S. pombe and will be achieved in the following manner: 1) Phenotypic characterization of the A4 mutant; 2) Clone and sequence the A4 gene; 3) Isolation of high copy suppressors of the A4 mutant; 4) Localization of the A4 gene product throughout the cell cycle; 5) Study the genetic interactions of A4 with other known components of the GTPase system. These studies will elucidate the role of the A4 gene in the pathway that controls the progress of a cell from mitosis to interphase of the cell cycle and enhance our knowledge of the in vivo function of the spil GTPase system.